Many burglar alarm systems have attempted to use ultrasonic sound to detect unauthorized intrusion, but have met with only qualified success. In attempting to maximize sensitivity to human intrusion, these systems have been too susceptible to false alarms, rendering them commercially undesirable. These false alarms are usually caused by electrical interference from power lines, electrical equipment, etc., and particularly in the case of ultrasonic alarms, to random background noise such as jet planes, auto traffic, etc., and to non-intrusive movement, such as air turbulence, hanging decorations, draperies and the like.
Another problem encountered by former ultrasonic systems was the difficulty, and therefore high cost, of installation. This was due to the unbalancing of the central alarm system as new rooms were added to the system requiring repetitive rebalancing of each room receiver with the central system as installation progressed.
IT IS THEREFORE AN OBJECT OF THIS INVENTION TO PROVIDE AN ULTRASONIC BURGLAR ALARM SYSTEM WHICH OPTIMALLY IS SENSITIVE TO INTRUSION WHILE GIVING NO FALSE ALARMS.
It is another object of this invention to employ a controlled wave pattern of ultrasonic radiation in an ultrasonic burglar alarm to increase the sensitivity of the alarm system.
It is a further object of this invention to provide an ultrasonic burglar alarm system which is simple and inexpensive to install and maintain.
It is a further object of this invention to provide an ultrasonic burglar alarm which employs transducers which are not affected by air turbulence.